1. Field of the Invention
The present invention relates to sequential data storage machines such as magnetic tape drives. More particularly, the invention concerns the modification of an implicit addressing sequential media drive by adding a hardware and/or software converter in order to simulate performance of an explicit addressing sequential media drive from the perspective of host applications.
2. Description of the Related Art
In the past, small computer system interface (SCSI) tape drives have used “stateless” commands. With stateless commands, also called “implicit addressing” commands, knowledge of current addressing is implied. Stated another way, an implicit Read, Write, or other position command consists of a command to start the operation at the current position without explicitly starting at a particular address. The host application or storage manager issues tape access and positioning commands that are relative. The following is an example of a series of implicit addressing commands: WRITE LENGTH 2, WRITE LENGTH 2, WRITE LENGTH 2. The components of each command include a write instruction and the number of units of data to be written.
Accordingly, it is crucial to keep track of the current write location, since the commands only specify a location relative to the last command. Many failures are difficult to recover from since the host application or device driver does not always know exactly where the drive is positioned after a failure. Accordingly, implicit addressing is limited because failed SCSI commands may leave the host without knowledge of the current tape position. Low level interconnect failures that are passed up to the command layer can result in failed backup jobs with retry of the entire job potentially taking hours, if possible at all. Another potential problem is that commands can be misordered by the time they reach the tape drive, causing the tape drive to apply them incorrectly.
Proposals are being developed for technical standards under which new tape drives will use explicit addressing. With explicit addressing, the host application or storage manager will issue tape access and positioning commands that specify the desired address. The following is an example of a series of explicit addressing commands that would carry out the same write sequence as the implicit write commands shown above: WRITE ADDRESS 0 LENGTH 2, WRITE ADDRESS 2 LENGTH 2, WRITE ADDRESS 4 LENGTH 2. Although explicit addressing will not offer random access, it will offer the advantage of making it easier to keep track of the location on tape that is being written. Explicit addressing will help avoid the situation where, as with implicit addressing, an inadvertent error in tracking the current tape location causes the storage subsystem to become confused as to where data is actually being written.
Although the explicit addressing tape drives are expected to constitute a significant advance, engineers at International Business Machines Corp. (“IBM”) are still researching different tape storage approaches to even better serve their customers. In this respect, IBM engineers have recognized that one byproduct of the evolution toward explicit addressing might be the abandonment of costly and otherwise useful implicit addressing drives. Host applications and intermediate storage systems designed to operate with explicit addressing tape drives are not compatible with the “legacy” implicit addressing tape drives. In some cases, customers may wish to purchase the latest storage subsystem but not abandon their legacy implicit addressing tape drives. For these customers, then, the state of the art is not completely adequate due to certain unsolved problems.